Income Taxes (Narrative) (Details) - USD ($) $ in Millions	3 Months Ended		12 Months Ended
	Mar. 31, 2018	Dec. 31, 2017	Mar. 31, 2019	Mar. 31, 2018	Mar. 31, 2017	Jun. 30, 2019	Mar. 31, 2016
Income Tax Disclosure [Line Items]
Effective Income Tax Rate Reconciliation, at Federal Statutory Income Tax Rate, Percent			21.00%	31.50%	35.00%
Income Tax Expense (Benefit)			$ 60	$ 406	$ 243
Tax Benefits Allocated Directly to Contributed Capital					65
Gross unrecognized tax benefits	$ 457		417	457	$ 389		$ 331
Amount of unrecognized tax benefits that would affect the effective tax rate			236
Combined amount of accrued interest and penalties related to uncertain tax positions	18		17	18
Amount of unrecognized tax benefits for which it is reasonably possible that there will be a reduction within the next 12 months			10
Deferred Tax Assets, Valuation Allowance	138		162	138
Deferred Tax Assets, Gross	249		296	249
Foreign Country [Member]
Income Tax Disclosure [Line Items]
Tax credit carry forwards			5
State [Member]
Income Tax Disclosure [Line Items]
Net operating Loss Carryforwards			598
Net operating loss attributable to various acquired companies			7
Tax credit carry forwards			$ 132
Subsequent Event [Member]
Income Tax Disclosure [Line Items]
Deferred Tax Assets, Gross						$ 2,300
Including U.S. Tax Cuts and Jobs Act impact [Member]
Income Tax Disclosure [Line Items]
Effective Income Tax Rate Reconciliation, at Federal Statutory Income Tax Rate, Percent			21.00%
Total U.S. Tax Cuts and Jobs Act impact [Member]
Income Tax Disclosure [Line Items]
Income Tax Expense (Benefit)	$ 59	$ 176		235
Transition tax [Member]
Income Tax Disclosure [Line Items]
Income Tax Expense (Benefit)				192
Increase to net cash provided by Operating Activities [Member] | Provision for income taxes [Member]
Income Tax Disclosure [Line Items]
Excess Tax Benefit from Share-based Compensation, Operating Activities			$ 20	$ 43